The invention relates generally to heat exchangers, and more particularly to a method and apparatus for controllable heat removal from a high-temperature process in order to maintain the process temperature within predetermined limits.
In many, if not all, high-temperature processes, the process temperature is optimally kept within certain limits. In certain high-temperature processes, relatively precise temperature control is necessary. One example of such a process is the thermal decomposition and oxidation of spent potlinings, which are generated in aluminum production, as explained in U.S. Pat. No. 4,763,585, which is incorporated herein by reference. Control of process temperature is important, because if the temperature is too low, combustion is incomplete, whereas if the temperature is too high, agglomeration results, which also leads to incomplete combustion. In combustion of spent potlinings in a fluidized bed reactor, it may be desirable for the combustion temperature to be maintained within a temperature range of, for example, 1500.degree. F. to 1550.degree. F.
In the past, attempts have been made to enable temperature control in fluidized bed combustion of spent potlinings by the use of a water-cooled bayonet tube heat exchanger. It has been found that temperature control is difficult to achieve merely by variation of water flow rate because of the large difference between the process temperature and the boiling point of the water. The water flow rate can only be reduced to the extent that the coolant temperature remains below the boiling point, because if the coolant were heated beyond the boiling point, unstable and unpredictable internal heat transfer conditions would occur along the lengths of the heat exchanger tubes. This would result in a lack of control over the heat removal rate, and additionally would impose unacceptably great thermal stresses on the tubes.
In one known reactor, temperature control has been addressed by enabling longitudinal movement of the bayonet tubes so that the tubes may be partially withdrawn from the process to reduce heat exchange. Due to the cost and mechanical complexity associated therewith, such mechanical variation of heat exchange surface area is not an entirely acceptable solution to the problem of temperature control. Another approach would be to subject the water to extremely high pressure, but this would, of course, increase the structural and pumping requirements of the system greatly.
There remains a need for an improved method and apparatus for removing heat from high-temperature processes such as fluidized bed combustion of spent potlinings.